Add and update type hints

[scaramallion]

• Type hints for event, fsm and _handlers
• Fixes for AA-7 and AA-8

Tasks

• Fix or feature added
• Unit tests passing and coverage at 100% after adding fix/feature

[codecov]

Codecov Report

Merging #630 (aa5f379) into master (0e8ac4b) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #630   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           27        27           
  Lines         8561      8624   +63     
=========================================
+ Hits          8561      8624   +63     

Impacted Files	Coverage Δ
pynetdicom/pdu_primitives.py	100.00% <ø> (ø)
pynetdicom/__init__.py	100.00% <100.00%> (ø)
pynetdicom/_handlers.py	100.00% <100.00%> (ø)
pynetdicom/acse.py	100.00% <100.00%> (ø)
pynetdicom/association.py	100.00% <100.00%> (ø)
pynetdicom/dimse.py	100.00% <100.00%> (ø)
pynetdicom/dimse_primitives.py	100.00% <100.00%> (ø)
pynetdicom/dul.py	100.00% <100.00%> (ø)
pynetdicom/events.py	100.00% <100.00%> (ø)
pynetdicom/fsm.py	100.00% <100.00%> (ø)
... and 2 more